The abovementioned various types of controls of braking force have heretofore been dealt individually by means of separate control units or separate systems each including an electronic control unit. However, such controls are possible by combining the increase, decrease and retaining of the brake fluid pressure both in braking and non-braking conditions. If one individual unit or system is allocated to each control factor, similar control elements have to be used for different controls, thus incurring such problems as economical loss, poor reliability, reduction in the space available in the vehicle and increase in the overall weight of the vehicle. An integrated system which can comprehensively deal all the controls would be very desirable.
In U.S. Pat. No. 3,667,813, there is disclosed a system which solves the abovesaid problem in the simplest way. In this system, a shut-off valve is provided in a piping line connecting a master cylinder with a wheel brake so as to open and close the brake fluid feed line. Also, between the shut-off valve and the brake are provided an access valve which electromagnetically changes over to selectively introduce or hold the pump pressure produced by a pump provided separately from the master cylinder and an electromagnetic discharge valve for selectively discharging or holding the fluid pressure so as to apply a pump pressure to the brakes and achieve a desired braking force. Besides, many similar systems are known in which an electromagnetic discharge valve is provided directly in a brake piping line. Also known are systems adapted to control the brake force indirectly by controlling the pressure in a control system.
In contrast, in a system in which a discharge valve is directly connected to a brake line, the malfunction of the discharge valve owing to worn sealing surfaces or foreign matters mixed in the brake fluid will directly lead to the loss of pressure on the brakes, thus bringing about a grave danger.
Many of the systems having a control line separate from the brake line are provided with various failsafe mechanisms so that the brake line will supply brake pressure even in case of the failure of a discharge valve disposed in the control line. But these systems have their drawbacks in that they structurally consume more brake fluid for the brake line and that the maximum attainable brake pressure is insufficient.
An object of the present invention is to solve the grave and critical problems of the prior art.